The present invention relates to a linear spring-loaded actuator for a valve.
Such a linear spring-loaded actuator comprises a housing in which an actuating spindle is slidably mounted. A cap screwed on a housing thread permits to preload a spring means arranged in the housing. This spring means comprises for example a cylindrical helical compression spring or, more often, a stack of Belleville spring washers. It bears on the cap and exerts a spring force on the spindle. The cap is for example screwed in the housing until it bears on a shoulder arranged below the housing thread. It is then fixed with a given tightening moment, thus ensuring the immobilization of the cap in the housing. A linear spring-loaded pneumatic actuator of this type is e.g. disclosed in EP-A-0 039 081.
The spring force of the actuator will have to be adjusted, depending for example on the valve specifications. This is generally done by inserting more or less washers between the cap and the spring means, so as to pre-load the spring means more or less. However, such an adjustment of the spring force with washers is rather complex and laborious. Indeed, the cap has to be removed, one or more washers have to be added or to be removed, and the cap has to be screwed in the housing and fastened with a given tightening moment. Moreover, washers do not allow to adjust the spring force with precision.
Consequently, there is a strong need for a linear spring-loaded actuator, wherein the spring force may be easily and precisely adjusted. According to the invention, this is achieved by a linear spring-loaded actuator according to claim 1.
In accordance with the invention a linear spring-loaded actuator for a valve comprises a housing and an actuating spindle slidably supported in the housing. A cap is associated with the housing. Spring means, arranged in the housing, bear on the cap and the actuating spindle, so as to exert a spring force on the actuating spindle. A thread on the housing cooperates with a thread on the cap for screwing the cap on the housing so as to preload the spring means in the housing. According to an important aspect of the invention, at least one lock means, associated with one of the threads, is capable of deforming this thread so as to lock it in any position on the cooperating thread. Hence, the presence of a shoulder is not required anymore to lock the two cooperating threads. In other words, the cap must not be screwed up to the depth of a shoulder to be immobilized in the housing. Indeed, thanks to the lock means, the thread associated thereto may be locked in any position on the cooperating thread. The cap may thereby be immobilized in any position on the housing thread. Varying the position of the cap on the housing thread allows to compress more or less the spring means in said housing. Hence, the spring force can easily be adjusted, simply by screwing in or off the cap on the housing thread. Moreover, the spring force may be adjusted with precision as the pre-load can be continuously adjusted. Once the desired spring force is obtained, the lock means is operated and the cap is immobilized.
Lock means may be associated with the housing thread or the cap thread or with both. In order to obtain a compact actuator it is however preferable to associate the lock means with the cap thread only. Such a cap is then preferably equipped with two diametrically opposed lock means associated with the cap thread.
A preferred lock means comprises a slot penetrating laterally in one of the threads so as to locally split the thread in two thread halves. In other words, the slot divides a first section from a second section. The first section has the first thread half thereon and the second section has said second thread half thereon. A screw is associated with the slot so as to be capable of locally drawing together or separating said two thread halves. Therefore, in any position of the cooperating threads, the thread halves may be pressed against the cooperating thread to cause their locking. In a first embodiment, the screw has a screw head and a screw barrel with a threaded foot end The screw head bears on a first of the two sections, and this first section and the slot are traversed by the screw barrel. The second section has a threaded bore in which the threaded foot end is received, so that it is possible to draw together the two sections by screwing in said screw. In a second embodiment, the screw has a threaded screw barrel and a foot end. A first of the two sections has a threaded through bore. The threaded screw barrel is screwed in the threaded through bore and traverses the slot. The foot end bears on the second section, so that it is possible to separate the two sections by screwing in the screw.
Preferably, the spring means comprises at least one Belleville spring washer.
A fluid operated piston assembly may be housed in the housing to overcome the spring force exerted by the spring means in order to actuate the actuating spindle.